In recent years, projectors along with personal computers are widely used for presentations in conference rooms and so on. They are also used as home theater projectors for home theaters in ordinary households.
Such projectors are designed to use light emitted from a light source to provide an enlarged projection of an optical image, the image processed in accordance with image information. As their light source, a high-pressure discharge lamp, in particular a high-pressure mercury lamp, which is closer to a point light source and has a high luminance and a high color rendering property, is widely in use.
The high-pressure mercury lamp has an arc tube inside which a light-emitting material, mercury of more than 200 mg/cm3, for example, is enclosed and a pair of tungsten electrodes is disposed substantially opposite to each other. Inside the arc tube, a halogen material, besides mercury, is also enclosed in order to use the so-called halogen cycle function to prevent tungsten, which is a constituent material of the electrodes, from diffusing and causing blackening by adhering to the inner wall of the arc tube during lighting.
It is known that, while the halogen cycle makes it possible to prevent the blackening of the inner wall of the arc tube, tungsten diffused from the electrodes again returns and adheres thereto, forming protuberances as it accumulates. Since the protuberances form bright spots of arcs between electrodes, when they are formed appropriately at tips of each electrode, stable arcs can be obtained so as to avoid occurrence of flicker due to so-called motions of arc bright spots.
In order to form and maintain such protuberances appropriately at the tips of each electrode, a high-pressure discharge lamp apparatus has been proposed, that includes a control unit for detecting shortening and lengthening of an electrode gap distance as the shapes of the protuberances change, and switching the frequency of an alternating current to be supplied to the high-pressure mercury lamp between a first value of less than 50 Hz or of 750 Hz and above, and a second value in a range from 50 to 700 Hz inclusive (Patent Document 1, for example). The high-pressure discharge lamp apparatus of Patent Document 1 is configured to promote growth of the protuberances by means of the alternating current of the second frequency when they reduce in size and the electrode gap distance increases, while promote evaporation by means of the alternating current of the first frequency when they grow and the distance decreases.
In contrast to the high-pressure discharge lamp apparatus of Patent Document 1, which is thus configured to recover the shapes of the protuberances after the shape changes took place, another high-pressure discharge lamp apparatus is also proposed, which is designed to modulate the frequency of an alternating current to be supplied to the high-pressure discharge lamp among two or more different values periodically (Patent Document 2, for example). The high-pressure discharge lamp apparatus of Patent Document 2 enables the protuberances to grow by means of an alternating current of at least one value and enables it to evaporate by means of an alternating current of the other values, and maintains the shapes of said protuberances by causing the growth and evaporation of the protuberances to repeat alternately.